In the summer of 2007, a large portion of Arctic Sea ice - about 40 per cent - simply vanished. That wasn't supposed to happen. At least not yet. As recent as 2004, scientists had predicted it would take another 50 to 100 years for that much ice to melt. Yet here it was happening today.

It raised the question: Had global warming suddenly pressed the gas pedal to the floor? If so, the world was in for quite a climate ride - dramatic, jarring changes in climate much sooner than expected. Climate scientists were deeply worried.

"It really caught the scientific community by surprise," Professor James Ford, a McGill University geographer and Arctic expert recalled. "The Arctic system is close to crossing the threshold beyond which we will get dramatic changes in climate."

The sudden mass melting brought an earlier ice event into new perspective. In 2005, scientists at the Canadian Ice Service, the nation's leading ice specialists, were examining satellite images when they noticed that the Ayles Ice Shelf, which is about as big as the island of Montreal, had suddenly broken free from the top of Ellesmere Island and floated away.

Vincent Warwick, an Arctic expert at Université Laval, said at the time: "This is a dramatic and disturbing event. It shows that we are losing remarkable features of the Canadian North that have been in place for many thousands of years. We are crossing climate thresholds, and these may signal the onset of accelerated change ahead."

The ice melt of 2007 seemed to confirm Warwick's fears. Reports since then claim the Arctic ice could be gone by 2013.

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We have already crossed some critical climate thresholds. The world not only has to drastically cut back its greenhouse gas emissions but also begin to take steps to deal with the inevitable changes that global warming will cause. The much-feared tipping points - which would cause massive icecap and ice shield melting, and plunge the world headlong into severe weather systems, causing broad devastation and rising seas - seem increasingly probable.

This is why, scientists say, the United Nations climate talks that began this week in Bonn, Germany, and will culminate in Copenhagen, Denmark, in December, are so important. They are a last chance for the world to come to its senses and negotiate an agreement to drastically cut greenhouse gas emissions.